Method of making colored photographs



Patented Sept. 8, 1942 vMETHOD OF MAKING'COLOEED PHOTOGRAPHS Alan M. Gundelfinger, Burbank, Callf., assignor to Cinecolor, Inc., Burbank, Calif., a corporation of California No Drawing. Application June 19, 1939,

Serial No. 279,915

3 Claims.

My invention relates generally to the field of color photography and more particularly to that branch of the art employing. the subtractive theory of color reproduction. Theprocess of my invention is applicable to both still and motion photography, but finds its greatest utility in the latter branch of the art.

It is an object of my invention to provide a novel method of coloring a developed silver image and of controlling that coloring so that adjacent images in other strata of the emulsion are not afiected thereby. 1 It is also an object of my invention to provide an improved method of mordantingan image so that better than ordinary results will be obtained when dye is applied to said mordanted image.

It is afurther object of my invention to provide an improved method of iodide mordanting a silver image, so that superior coloring may be had and staining of the gelatin kept to a minimum.

These and other objects of my invention will become readily apparent from the following description of a preferred form of my invention, which is given by way of example to illustrate the principles involved.

Since my invention finds its greatest utility in color cinematography, I will describe a complete color process particularly well adapted for this branch of the art, it being understood of course that other applications and adaptations can be made of my invention.

In producing colored positives for projection from color separation negatives, it is desirable to use a film having an emulsion on but one side and to superpose the separate color-value part positive images indifferent strata of said emulsion. This can readily be done by printing from one color-value negative to the face of the positive stock having an emulsion dyed with a water soluble yellow dye to restrain penetration of light, and printing from a different color-value negative through the base of the positive stock-to produce a second image inthe inner stratum of the emulsion. If a three-color process is being practiced, a third image may be produced.- in a number of convenient ways. When the two latent images have been printed in superposed strata, they are developed'and fixed to produce complemental silver images.

Normally, when following a process of this type, the two images are quite close to each other, since the emulsion layer is preferably very thin. Consequently, the task of treating the outer image without contaminating the inner image is always considerable of a problem, and particularly where it is desired to mordant'the outer image with iodide or some other suitable mordant. As mentioned, it is one of the objects of my invention to provide a method of mordanting whereby this outer image can be satisfactorily mordanted without also mordanting the innerimage, although, it will be'understood that my invention is not to be limited to this situation.

After the superposed complemental images have been developed to free silver, the outer image is preferably oxidized to a silver salt having a solubility product equal to or greater than silver iodide. This salt is preferably a-halide such as silver chloride. Lprefer to accomplish this oxidation by treating the film with an alcoholic solution of aferric halide such as ferric chloride or ferric bromide, but preferably the former. For best results, the alcohol should be ethyl or methyl Ferric chloride l 'grams 200 Nitric acid (cone). ccl 1 Alcohol to make lit'er 1 or'other form of alcohol having four carbonsor less. A formula which I have found to be very satisfactory for this oxidizingst'ep is as follows:

I have found that the, rate of penetration of this solution is readily controlled and that treatment .of the film for two or three minutes results'in a substantially complete conversion of the silver in the outer image to silver chloride, without affecting the inner image.

After removal from the bleaching 'bath, the film is preferably immersed for a few minutes'in a solution such as potassium oxalate e. g.a ,2% solution, in which a small quantity of salt such as sodium chloride has been dissolved. The ex-- alate reduces the concentration of ferric ions by forming a complex and prevents the hydrolysis of the ferric chloride in the film with the subsequent formation of ferric hydroxide. The high concentration ofv chloride ions present by" reason of the added salt suppresses the ionization of-the silver chloride and thus prevents any redevelopment by the. ferrous salt present in the film.

After a water wash, the film, now carrying an inner silver image and an outer silver chloride image. is subjected to the action of a solution which will convert the silver chloride image to silver iodide. A suitable bath for this purpo may be made up'as follows:

Potassum iodide grams Potassium alum ....do 14 Sodium aceta j do 5 Acetic acid 5 Water to make" -Jiter-.. 1

The film is left'in this solution until the outer image has been substantially 'completelyconverted to silver iodide, or rather to a potassium silver iodide gelatin complex which is an active mordant for basic dyes. v

Upon being removed from this solution, the

film is preferably washed and dyed with a suit able dye, and is washed again. The inner image may then be colored a complementary color by any one of various methods as for instance iron toning to produce a blue if the outer image has been dyed red.

By this procedure of employing what I may term a two-step method of mordanting the outer image, I am able to satisfactorily control the mordanting of the outer image and not contaminate the inner image, whereas with conventional methods, it is practically impossible to accurately control the penetration of the mordanting solution. With the conventional iodine bleach, the rate of penetration is so rapid that accurate control is impossible even when employing a I quick acting stop bath. However, in the process herein disclosed, the alcoholic solution of an oxidizer such as ferric chloride is slow penetrating and can be. easily controlled. Then when the outer image has been converted to a substance diiferent from that of the inner image, the necessity of controlled penetration ceases to be a problem.

In practicing the foregoing process, I have discovered that the pH or hydrogen ion concentration of the solution used to convert the silver salt image to silver iodide plays an important part in the quality of the resulting product, and I have determined that there are certain critical limits for the pH of this treating solution. These results are substantiated both theoretically and experimentally, and I have found them to be quite important.

It is my theory, as discussed in detail in the patent, to Gundelfinger No. 2,141,354, dated December 27, 1938, that the resulting mordant is not merely silver iodide as would be expected from an inspection of the chemical reactions between the various ingredients of the bath and the silver image, but is a complex or plurality of complexes having the general formula KyAgeI(:r+y), (where z: and y denote numerical subscripts and potassium is used as an example of an alkali metal, including the NH. group), which under certain conditions mordants to gelatin to probably form what; may be termed an alkali-metal silver iodide gelatin complex or agglomerate, which is electrostatically negative and mordants basic dyes. The amount of dye which will be absorbed by the image depends of course on the amount of this complex present. Since the alkali-metal silver iodide complex is electrostatically negative and soluble, if the gelatin is not maintained in an electrostatically positive condition, the said complex escapes into the treating solution as it is being formed.

I have discovered that the transparency and mordanting power of the image increases as the pH of the treating solution decreases, i. e. as the hydrogen ion concentration increases. This indicates that the gelatin is probably put in a more positive electrostatic condition by the presence of the positively-charged hydrogen ions, and can thus hold more of the electrostatically negative metallic salt complex and consequently mordant more dye to the image. I have determined that the permissible upper limit for the'pH of the treating solution without a bufler is approximately 4.0, and the optimum value is somewhat lower than this. With a pH higher than 4.0,

the quality of the image is such as to make it commercially unsatisfactory. If a bufler such as sodium acetate is used, the solution can be maintained at a higher pH value and still obtain satisfactory results, and at any given pH, the image quality is better than with the unbuflered solution. This indicates that in th6-KyA8::I($+1I) gelatin complex formation hydrogen ions are consumed and the resulting complex or agglomerate must consist of a combination of KvAg=I(:c+1/) plus gelatin plus H+. When the solution is buffered, the pH can be maintained as high as 5.0 and still get satisfactory quality, although as before the optimum value is below this point.

It is my belief that the beneficial effects derived from decreasing the pH of the treating solution are due to the increase in the electro positive charge of the gelatin bythe addition of the positive charge contributed by the hydrogen ions, and this is borne out by the buffer action above mentioned which makes a greater number of H ions available in the gelatin for a given pH value. Further proof of the correctness of this theory is derived from the fact that when tri-valent ions from alum or other substances which do not oxidize potassium iodide are added to the solution a still better quality of image is obtained for a given pH value, this being true in both the buffered and unbuflered solutions. The pH must of course be kept low enough so thatthe tri-valent ion producing substance will stay in solution, which for alum, is about pH 4.1. Some other suitable tri-valent substances are potassium alum, chrome-alum, aluminum sulphate and chromium sulphate, all of which exist only at a pH below approximately 5.0.

It appears that the tri-valent ions go into the gelatin and subsequently the complex just as the H ions do but a proportionately greater increase in mordanting power is obtained since each such ion contributes three positive charges whereas the H ions of course can only contribute one positive charge. ,A particular advantage in the use of alum is that it is itself a buffer and hence is doubly effective.

As previously mentioned, the upper limits of the pH value of the treating solution are critical and can be quite accurately fixed, From the standpoint'of good transparency and mordanting power, it is desirable that the pH be as low as possible, since, as previously shown, these qualities increase with decrease in pH value of the treating solution. However, from a practical standpoint, the pH cannot be lowered too much. Furthermore, the lower limit depends on many variable factors and no accurate value can be established for all films and all treating conditions.

Broadly speaking, it can be said that the lower limit for any particular film depends upon the ability of its gelatin to withstand reticulation and to keep its proper consistency, i. e. not get too soft. These qualities, of course, vary with diflerent films and also depend upon the characteristics and concentrations of the various substances used in the treating solutions, the temperature of the solution and various other factors. For example, practically all salt solutions tend to reticulate the film, as do acids, and a given film can only stand somuch, which amount increases as the temperature is decreased. Likewise, some salts because of their inherent characteristics are harder on emulsions than others. Furthermore, the ability of the film to withstand reticulation can be increased by employing certain hardeners such as formaldehyde,

alum, and the like. In this connection, it is to be noted that alum thus has a third function or advantage, 1. e. it suppresses reticulation as of 2.5 can safely be employed with mostfilms now on the market and under ordinary treating conditions, although as stated this allowable lower value depends upon numerous variables. It is, of course, not wise from a practical standpoint to operate too close to any limits and I therefore recommend apI-I between approximately 3.0 and 3.8, depending upon whether a buffer and/or a tri-valent ion is used in the solution.

A modified form of my invention involves th substitution of suitable metals such as mercury, gold, platinum, and the like for silver in the resulting complex mordant, and I have found that the pH control and tri-valent ion addition.

are equally beneficial when using this form of my invention.

Two silver images may be obtained in the usual manner as previously described, and the outer one treated with the proper salt of the metal which is to be substituted for the silver. A suitable oxidizing salt of this nature is mercuric chloride (if mercury is to be used), which oxidizes the silver to silver chloride and is itself reduced to mercurous chloride. The silver chloride is then removed by a suitable solvent such as thiosulphate which also reduces the mercurous chloride to metallic mercury.

From this point on, the process follows the steps previously described, the mercury image being oxidized back to mercurous halide by a ferric halide and then converted to a KyHg=I(a:+y) gelatin complex with added I-I ions and/or valent ions.

tri-

While I have disclosed the preferred form of my invention as applied to the production of a color photograph made up of superposed part images and have pointed out the value of my twostep method of producing a mordanting image in the outer stratum of the emulsion layer, it is to be understood that the pH control feature of my invention, i. e. maintaining the pH below the limits hereinbefore given, and'the feature of my invention. which involves the addition of trivalent ions to the emulsion, are also applicable to, and produce comparably beneficial results when mordanting single images.

I claim as my invention:

1. The method of coloring a silver photographic image which includes: oxidizing said image to silver chloride by treatment 'with aferric chloride solutio'n;\converting said silver chloride image to silver iodide by treatment with an alkali-metal iodide solution; controlling the pH of said iodide solution so as-to maintain it below 4.0; and dyeing the resulting iodide image.

2. The method of coloring a silver photographic image which includes: oxidizing said image to silver chloride by treatment with a ferric chloride solution; arresting the action of said oxidizer by treating said image with a solution of an oxalate and a chloride; converting said silver chloride image to silver iodide by treatment with an alkali-metal iodide solution; controlling the pH of said iodide solution so as to maintain it below 4.0; and dyeing the resulting iodide image.

3. The method of coloring a silver photographic image which includes: oxidizing said image to silver chloride by treatment with a ferric chloride solution; converting said silver chloride image to silver iodide by treatment with an alkali-metal iodide solution containing trivalent positive ions from a salt chosen from the group consisting or alum, potassium alum, chrome-alum, aluminum sulphate and chromium sulphate; controlling the pH of said iodide solution so as to maintain it below 5.0; and dyeing the resulting iodide image.

ALAN GUNDELFINGER. 

